352 research outputs found
InGaAs/InP double heterostructures on InP/Si templates fabricated by wafer bonding and hydrogen-induced exfoliation
Hydrogen-induced exfoliation combined with wafer bonding has been used to transfer ~600-nm-thick films of (100) InP to Si substrates. Cross-section transmission electron microscopy (TEM) shows a transferred crystalline InP layer with no observable defects in the region near the bonded interface and an intimately bonded interface. InP and Si are covalently bonded as inferred by the fact that InP/Si pairs survived both TEM preparation and thermal cycles up to 620 °C necessary for metalorganic chemical vapor deposition growth. The InP transferred layers were used as epitaxial templates for the growth of InP/In0.53Ga0.47As/InP double heterostructures. Photoluminescence measurements of the In0.53Ga0.47As layer show that it is optically active and under tensile strain, due to differences in the thermal expansion between InP and Si. These are promising results in terms of a future integration of Si electronics with optical devices based on InP-lattice-matched materials
Revealing large-scale homogeneity and trace impurity sensitivity of GaAs nanoscale membranes
III-V nanostructures have the potential to revolutionize optoelectronics and
energy harvesting. For this to become a reality, critical issues such as
reproducibility and sensitivity to defects should be resolved. By discussing
the optical properties of MBE grown GaAs nanomembranes we highlight several
features that bring them closer to large scale applications. Uncapped membranes
exhibit a very high optical quality, expressed by extremely narrow neutral
exciton emission, allowing the resolution of the more complex excitonic
structure for the first time. Capping of the membranes with an AlGaAs shell
results in a strong increase of emission intensity but also to a shift and
broadening of the exciton peak. This is attributed to the existence of
impurities in the shell, beyond MBE-grade quality, showing the high sensitivity
of these structures to the presence of impurities. Finally, emission properties
are identical at the sub-micron and sub-millimeter scale, demonstrating the
potential of these structures for large scale applications.Comment: just accepted in Nano Letters,
http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b0025
Nanometer-scale sharpness in corner-overgrown heterostructures
A corner-overgrown GaAs/AlGaAs heterostructure is investigated with
transmission and scanning transmission electron microscopy, demonstrating
self-limiting growth of an extremely sharp corner profile of 3.5 nm width. In
the AlGaAs layers we observe self-ordered diagonal stripes, precipitating
exactly at the corner, which are regions of increased Al content measured by an
XEDS analysis. A quantitative model for self-limited growth is adapted to the
present case of faceted MBE growth, and the corner sharpness is discussed in
relation to quantum confined structures. We note that MBE corner overgrowth
maintains nm-sharpness even after microns of growth, allowing the realization
of corner-shaped nanostructures.Comment: 4 pages, 3 figure
Position controlled self-catalyzed growth of GaAs nanowires by molecular beam epitaxy
GaAs nanowires are grown by molecular beam epitaxy using a self-catalyzed,
Ga-assisted growth technique. Position control is achieved by nano-patterning a
SiO2 layer with arrays of holes with a hole diameter of 85 nm and a hole pitch
varying between 200 nm and 2 \mum. Gallium droplets form preferentially at the
etched holes acting as catalyst for the nanowire growth. The nanowires have
hexagonal cross-sections with {110} side facets and crystallize predominantly
in zincblende. The interdistance dependence of the nanowire growth rate
indicates a change of the III/V ratio towards As-rich conditions for large hole
distances inhibiting NW growth.Comment: 9 pages, 4 figure
The role of hydrogen in the formation of microcrystalline silicon
The growth mechanisms of microcrystalline silicon thin films at low temperatures (100-250°C) by plasma CVD are still a matter of debate. We have shown that ue-Si:H formation proceeds through four phases (incubation, nucleation, growth and steady state) and that hydrogen plays a key role in this process, particularly during the incubation phase in which hydrogen modifies the amorphous silicon network and forms a highly porous phase where nucleation takes place. In this study we combine in-situ ellipsometry and dark conductivity measurements with ex-situ high resolution transmission electron microscopy to improve our understanding of microcrystalline silicon formation
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Exciton localization mechanisms in wurtzite/zinc-blende GaAs nanowires
We investigate the emission properties of excitons in GaAs nanowires containing quantum disks formed by structural alternation between the zinc-blende and wurtzite phases, by means of temperature-dependent photoluminescence. At 10 K the emission from an ensemble of disks is distributed in a band of full width at half maximum âŒ30 meV, whereas the emission linewidth for a single disk is 700 ÎŒeV. While the disk ensemble emission exhibits an S-shaped temperature dependence, the emission from single quantum disks follows the temperature dependence of the band gap over the whole temperature range. This indicates that intradisk exciton localization on impurities is negligible and that increasing the temperature induces a transfer of excitons from narrow to thick disks along the length of the wires. Our observations of the emission linewidth for single crystal-phase quantum disks show a scattering rate of excitons with acoustic phonons eight times larger than the values usually reported for (Al,Ga)As/GaAs quantum wells. This large scattering rate demonstrates that the electron effective mass in wurtzite GaAs is much heavier than in zinc-blende GaAs and is evidence of coupling between the Î7 and Î8 conduction bands of wurtzite GaAs.We acknowledge financial support from the Poynton
Cambridge Australia Scholarship and from the European
Union Seventh Framework Program under grant agreement
No. 265073. A.F.iM. and E.U. acknowledge funding through
the Marie Curie Excellence grant SENFED. S.C.B. thanks
S.N.F. for funding through the Marie-Heim Vögtlin scheme
Growth study of indium-catalyzed silicon nanowires by plasma enhanced chemical vapor deposition
Indium was used as a catalyst for the synthesis of silicon nanowires in a plasma enhanced chemical vapor deposition reactor. In order to foster the catalytic activity of indium, the indium droplets had to be exposed to a hydrogen plasma prior to nanowire growth in a silane plasma. The structure of the nanowires was investigated as a function of the growth conditions by electron microscopy and Raman spectroscopy. The nanowires were found to crystallize along the , or growth direction. When growing on the and directions, they revealed a similar crystal quality and the presence of a high density of twins along the {111} planes. The high density and periodicity of these twins lead to the formation of hexagonal domains inside the cubic structure. The corresponding Raman signature was found to be a peak at 495 cmâ1, in agreement with previous studies. Finally, electron energy loss spectroscopy indicates an occasional migration of indium during growt
Tuning growth direction of catalyst-free InAs(Sb) nanowires with indium droplets
The need for indium droplets to initiate self-catalyzed growth of InAs nanowires has been highly debated in the last few years. Here, we report on the use of indium droplets to tune the growth direction of self-catalyzed InAs nanowires. The indium droplets are formed in situ on InAs(Sb) stems. Their position is modified to promote growth in the or equivalent directions. We also show that indium droplets can be used for the fabrication of InSb insertions in InAsSb nanowires. Our results demonstrate that indium droplets can initiate growth of InAs nanostructures as well as provide added flexibility to nanowire growth, enabling the formation of kinks and heterostructures, and offer a new approach in the growth of defect-free crystals
Local modification of GaAs nanowires induced by laser heating
GaAs nanowires were heated locally under ambient air conditions by a focused laser beam which led to oxidation and formation of crystalline arsenic on the nanowire surface. Atomic force microscopy, photoluminescence and Raman spectroscopy experiments were performed on the same single GaAs nanowires in order to correlate their structural and optical properties. We show that the local changes of the nanowires act as a barrier for thermal transport which is of interest for thermoelectric applications
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